1. Field of the Invention
The present invention relates to optical transmission systems; more specifically, wavelength stabilization for multichannel optical transmission systems.
2. Description of the Prior Art
In multichannel optical transmission systems (WDM-Systems) it is presently possible to discern a trend towards constantly higher numbers of channels. Since the optical fibers and amplifiers used for signal transmission have good transmission properties for optical signals only with a certain wavelength or located within a certain range of wavelengths, the channel separations between the individual carrier signals diminish to the extent that the number of channels increases. The reduction of channel separations results in higher requirements for the stabilization of the wavelengths of the individual carrier signals or the sources of laser light generating them.
In order to compensate for changes of wavelength of the carrier signals due to variations of the temperature of the surroundings or the alteration of the source of laser light, various adjustments of wavelength or of frequency have become known.
From the article xe2x80x9c16-channel Optical FDM Distribution Transmission Experiment Utilizing Multichannel Frequency Stabilizer and Waveguide Frequency Selection Switchxe2x80x9d by H. Toba et al., published in Electronics Letters, Apr. 27, 1989, vol. 25 No. 9, pp. 574-576, a wavelength stabilization [method] is known in which low frequency signals modulate the carrier signals by means of frequency modulation. By means of an optical ring resonator the individual carrier signals are regained from the total optical signal. The regulation of wavelengths for the individual carrier signals occurs through analysis of the frequency shift of the carrier signal due to modulation applied by the low frequency signals. The regulation of the wavelength for the individual carrier signals occurs through analysis of the frequency shift of the carrier signal caused by the modulation applied by low-frequency signals to the carrier signal. To this end the optical signal at the output of the ring resonator is transformed into an electrical signal, and the low-frequency signals contained in it are regained through demodulation.
In the article xe2x80x9cWDM Coherent Star Network With Absolute Frequency Referencexe2x80x9d by Y. C. Chung et al., published in Electronics Letters, Oct. 13, 1988, vol. 24 no. 21, pages 1313-1314, a wavelength stabilization [method] by means of a reference laser light source is described.
Yet the known wavelength stabilization [methods] have the disadvantage that the extraction of the regulating signals required for the wavelength stabilization of the individual optical carrier signals require a great expenditure, because additional optical components are required. For the first known wavelength stabilization [method] an optical frequency discriminator is required, and for the second known wavelength stabilization [method] a reference laser is necessary. Such optical constituent parts cause additional expenditures.
Thus it is the purpose of the invention to indicate a method and an arrangement for stabilizing the wavelengths of multi-channel optical transmission systems in which there exists for every channel a transmitter with a source of laser light which generates an optical carrier signal with a wavelength predetermined for each respective channel, in which the disadvantages of the known state of the art are avoided. Especially a wavelength regulation [method] at a reduced cost is to be made possible.
According to the invention, wavelength stabilization of the individual optical carrier signals is made possible by the fact that a certain low-frequency signal is impressed on the individual channels by means of frequency modulation and/or amplitude modulation, and that the signals of the individual channels of the optical transmission system are combined by means of a wavelength selective optical multiplexer into a total optical signal. In order to impress the low frequency signal on the optical carrier signal, the laser injection current is slightly modulated ( less than 1%). By changing the injection current there originates a frequency modulation as well as an amplitude modulation of the optical carrier signal. Both kinds of modulation can be analyzed for the regulation of the wavelength stabilization. For the analysis the total optical signal is transformed into an electrical signal, and the low frequency signals of the individual channels are regained. By analysis of the amplitudes of the individual low-frequency signals, wavelength stabilization of the individual carrier signals is possible.
Through the use of a wavelength selective optical multiplexer, which is normally used in multichannel optical transmission systems having a large number of channels for forming the total optical signal, a modification of the amplitude of the optical carrier signal for the channels results, for those carrier signals showing a deviation from the desired wavelength. A condition for this is the use of wavelength selective multiplexers with an absolute maximum or a local minimum in the channel midpoint, such as e.g. with phased arrays. By the amplitude modulation of the individual carrier signals with various low frequency signals moreover, a low frequency electrical analysis of the change of amplitude via a wavelength deviation is possible, which also has an effect on the low-frequency signals.
The advantage of the invention consists especially in the fact that for the realization of the wavelength stabilization no expensive and high-priced optical components such as frequency discriminators or reference lasers are needed. In addition, through the use of low frequency signals for the determination of the frequency deviation of the optical signals a simple electrical analysis is made possible.